Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching similar to those developed for the fabrication of integrated circuits.
Digital micromirror devices (MDs), sometimes referred to as deformable micromirror devices, are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and DMDs have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the DMD is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, DMDs typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Thin hinge structures, which restrict the deformation to a relatively small region of the device, limit the amount of light scattered and improve image quality.
Torsion beam devices enabled the use of dark field optical systems. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface. The rigid member or mirror is suspended by, and typically centered on, the torsion hinge-allowing the mirror to rotate by twisting the torsion hinge. Address electrodes are formed on the substrate beneath the mirror on either side of the torsion hinge axis. Electrostatic attraction between an address electrode and the mirror, which in effect form the two plates of an air gap capacitor, is used to rotate the mirror.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by using an elevated mirror to block most of the light from reaching the torsion beam hinges. The elevated mirror is connected to an underlying torsion beam or yoke by a support post. The yoke is attached to the torsion hinges, which in turn are connected to rigid support posts. Because the structures that support the mirror and allow it to rotate are underneath the mirror instead of around the perimeter of the mirror, virtually the entire surface of the device is used to fabricate the mirror. Since virtually all of the light incident a hidden-hinge micromirror device reaches an active mirror surface-and is thus either used to form an image pixel or reflected away from the image to a light trap-the hidden-hinge device's contrast ratio is much higher than the contrast ratio of previous devices.
Images are created by positioning the DMD so that a light beam strikes the DMD at an angle equal to twice the angle of rotation. In this position, the mirrors fully rotated toward the light source reflect light in a direction normal to the surface of the micromirror device and into the aperture of a projection lens-transmitting light to a pixel on the image plane. Mirrors rotated away from the light source reflect light away from the projection lens-leaving the corresponding pixel dark.
Full-color images are generated either by using three micromirror devices to produce three single-color images, or by sequentially forming three single-color images using a single micromirror device illuminated by a beam of light passing through three color filters mounted on a rotating color wheel.
Micromirror devices are generally operated in one of two modes of operation. The first mode of operation is an analog mode, sometimes called beam steering, wherein the address electrode is charged to a voltage corresponding to the desired deflection of the mirror. Light striking the micromirror device is reflected by the mirror at an angle determined by the deflection of the mirror. Depending on the voltage applied to the address electrode, the cone of light reflected by an individual mirror is directed to fall outside the aperture of a projection lens, partially within the aperture, or completely within the aperture of the lens. The reflected light is focused by the lens onto an image plane, with each individual mirror corresponding to a fixed location on the image plane. As the cone of reflected light is moved from completely within the aperture to completely outside the aperture, the image location corresponding to the mirror dims, creating continuous brightness levels.
The second mode of operation is a digital mode. When operated digitally, each micromirror fully deflects in either of the two directions about the torsion beam axis directing the entire cone of reflected light either inside or outside the aperture of the projection lens. Thus, the digital mode either creates a maximum-brightness pixel or a minimum-brightness pixel. Intermediate brightness levels are created by pulse width modulation techniques in which the mirror is rapidly and repetitively rotated on and off. The duty cycle of the mirror determines the quantity of light reaching the image plane. The human eye integrates the light pulses and the brain perceives a flicker-free intermediate brightness level.
Digital operation uses a relatively large voltage to ensure the mirror is filly deflected. Since it is advantageous to drive the address electrode using standard logic voltage levels, a bias voltage, typically a negative voltage, is applied to the mirror metal layer to increase the voltage difference between the address electrodes and the mirrors. Use of a sufficiently large mirror bias voltage-a voltage above what is termed the threshold voltage of the device-ensures the mirror will deflect to the closest landing electrodes even in the absence of an address voltage. Therefore, by using a large mirror bias voltage, standard logic level address voltages may be used.
Typical images do not result in a 50/50 on/off duty cycle for the average micromirror. The unequal loading of the on and off states of a pixel results in a time average angle which produces a permanent deformation of the hinge. This deformation is seen as a permanent torsion, or twist, in the micromirror hinges. In the worst case, the hinge twist, or hinge memory, becomes so large that the force created by the address electrodes is insufficient to overcome the permanent twist. Long before the hinge twist is able to overcome the force generated by the address electrodes, however, the hinge twist degrades the image quality by occasionally overcoming the force generated by the address electrode.
When creating an image with the micromirror device, the micromirrors are rapidly switched on and off. Due to the resonant reset process used to ensure the micromirror releases from the surface of the landing electrodes, un-addressed mirrors tend to flutter about the neutral, or flat, position. Likewise, an addressed mirror flutters about the position determined by the address voltages-a position also influenced by hinge memory. Therefore, the hinge memory and the mirror flutter combine to overcome the force generated by the address electrodes and, depending on the timing of the mirror bias signal relative to the position of the fluttering micromirror, can intermittently cause improper device operation long before the hinge memory becomes large enough to consistently limit mirror rotation. Current micromirror designs are unacceptable when the hinge deformation produces a 4 degree deviation from the flat state.
The effects of hinge memory can be mitigated by the use of larger address voltages and by allowing the fluttering micromirror to settle before applying the mirror bias voltage. Unfortunately, each of these alternatives has a detrimental impact on the design and operation of micromechanical devices. Therefore, a method of eliminating or reducing hinge memory is needed.